Light emitting devices, such as lasers, have been used as a sensor component to gather information in various applications. For example, time of flight measurement apparatuses, such as laser scanners and light detection and ranging apparatuses (hereinafter referred to as “LIDAR”), have been used for many applications. Examples of such applications include terrain mapping, bathymetry, seismology, detecting faults, biomass measurement, wind speed measurement, Differential Absorption LIDAR (DIAL), temperature calculation, traffic speed measurement, military target identification, surface to air rangefinding, high definition surveying, close range photogrammetry, atmospheric composition, meteorology, distance measurement, as well as many other applications.
LIDAR has been increasingly used for surveying and topographical mapping of geographical areas, for example, using downward-looking LIDAR instruments mounted in vehicles, such as aircraft or satellites. Such LIDAR instruments are used to determine the distance to a surface, such as a surface of an object or a terrain surface, using pulses of light. The range to the surface is determined by measuring the time delay between transmission of a pulse of light and detection of a corresponding reflection signal. In such systems, speed of light is used as a known constant for calculating the distance using the time of light travel.
Airborne LIDAR systems have been used for direct measurement of ground surfaces and natural and man-made objects from the air. Typically, the data, such as laser range and return signal intensity measurements, is recorded by a LIDAR system in-flight, along with positional and attitude data derived from airborne GPS and inertial subsystems. Data models generated can include high spatial resolution “point clouds” that can also yield details under tree cover and provide “bare earth” terrain models used for orthorectification of aerial imagery (using standardized software packages). As the aircraft flies across the project area, pulses of light are fired toward the ground one after another at a high rate. These pulses of light are reflected by the ground and/or objects upon the ground such as trees and buildings.
Laser scanners are also used to obtain data models describing surfaces, such as surfaces of objects. One example of a laser scanner is disclosed in U.S. Pat. No. 6,734,849, the contents of which are incorporated herein by reference. A laser scanner, such as the laser scanner disclosed in U.S. Pat. No. 6,734,849, can be used to collect points to form a point cloud describing the scanned surface.
According to these conventional embodiments, a subsequent pulse of light is not sent until a return reflection signal from the previous pulse of light is received. For each pulse of light, the elapsed time between the emitted and returning signals is measured, which enables a vertical, or a slant distance, to be computed. The location of the reflective surface can be calculated based on: (1) the angle with respect to the system at which the pulse of light is transmitted, (2) the orientation of the system with respect to the earth and (3) the current location of the system. As the measurements progress, data from such laser firings, often numbering in the millions, can be captured and additional data models describing the reflecting surface can be recorded, providing a dense digital terrain model (DTM) or digital surface model (DSM), for example. However, these conventional embodiments have been limited as to the rate at which pulses of light can be sent and received.